USF is a family of transcription factors that recognize similar DNA- binding sites as the Myc oncoproteins. Two USF family members, USF1 and USF2, are ubiquitously expressed in both human and mouse. In vitro studies indicate an involvement of USF in growth control. For example, USF1 and USF2 are both capable of antagonizing the transforming ability of c-Myc. Also, the levels of USF1 and USF2 vary during the cell cycle and their transcriptional activity is modulated by different cyclins. Finally, a complete loss of USF function was demonstrated in a number of cancer cell lines, indicating that the lack of USF confers a distinct growth advantage in certain cellular context. In order to establish the biological function of the USF proteins, targeted mutations were individually induced in the Usf1 and Usf2 genes by homologous recombination in murine embryonic stem cells. These experiments revealed that the USF proteins affect diverse biological pathways, since mice containing a disrupted Usf1 gene display altered brain function, while mice lacking Usf2 are deficient in growth. Analysis of the USF expression in these mice also demonstrated the existence of a feedback mechanism that normally regulates both the combined levels and the relative abundance of the different USF dimers. To further investigate the involvement of USF in growth control, experiments are proposed that will generate animals with combined mutations in both Usf genes. Together with the single mutations, these USF-null mice will constitute a powerful model system to determine the individual and combined functions of the USF proteins in vivo and the significance of their crossregulation. Matched sets of genetically defined cell lines will be derived from the different USF-deficient mice and used to determine the individual and combined roles of USF1 and USF2 in cell cycle control, proliferation, and transformation. These cell lines will also permit unambiguous analysis of the roles of USF1 and USF2 in the expression of suspected target genes.